Procedure, Of the finely powdered steel and of a. typical steel, o-i gram
is introduced into each of two thick walled test tubes (r^j mm. high and
15 mm. in diam.), Into each tube 5 c.c. of nitric acid (I) i-iS-i-a) free
from liydrocliloric acid are poured, the tube being covered with a. funnel
and immersed in a, bath of cold water to allay the vigour of the reaction.
After a short time the tubes are transferred to a vessel of water and heated
at (So -100"l for r-j hours, care being taken that the level of liquid in the
bath is not more than about i cm. higher than that in the tubes,
After i -„•! hours' heating, when the action is at an end, the.
tubes are cooled in cold water and the solutions poured into
white glass tubes of the same bore and thickness, holding 30
cm. and divided into tenths of i c.c. (Kggert/ tubes, ,svc Fig.
I,;), filtering it necessary (that is, if much silica remains)
through a small asbestos filter away from direct light
To carry out the colorimetric comparison the volume of
each solution is made up to S c.c , in order that the colour may
not be influenced by (hat ol the dissolved iron or impurities
and the colours viewed by transmitted and reflected light.
The darker one is then carefully diluted with water until the
intensities correspond ; the tubes should be interchanged and
the comparison repeated,
The comparison is facilitated by the use of a box with
opaque, black, lateral walls, the back one sliding, up to admit
of examination by transmitted light. The combined carbon is
deduced from the proportion.
where I" and •,« are the respective volumes of solution from the
,. control steel and the steel under examination and f' and ;v the.
corresponding contents of combined carbon.
With practice the t'litiii'htti'h'ii method c.ives sullicienlly rxuct results and is
rapid and permits of several simulla.iieous determinations, ft is naturally of
advantage to compare steels of the same kind, e.f,,, a Mart in steel with another
Martin steel and a. Bessemer steel with a Bessemer si eel, and I he control steel the
carbon-content of which should In- determined very accurately bv combustion •
should have about the same content of carbon us. or better a, rather greater
content than, the steel to be examined, in general it is sullicient, to have a
series of control steels containing approximately ; o-oo, o-i,-, oM;,, os'.o, 0-30,
0-50, o-Ho and is^u% of carbon. If the steel has a carbon mob-til greater than
o'S%, the calculation may be simplified by dilutine, the solution of the control
steel to such a. volume that the number expressing the c.c. i orresponds with the,
carbon-content, of the steel (the solution of a control steel containing1,, say, 0-0,%
of carbon would be diluted to o e.e.}. When the solution trout the steel to lie
tested has the same colour as that from the control steel, its volume divided by
,10 gives at once the perei nlu|»,e id' carbon.
The t'liliii'iiiK'ti'ic method cannot be used (or steels coitlainhu', melals capable
of modifying appreciably the coloration of the nitric acid solution, such as
chromium, copper, nickel, etc.
1 The temperature is easily maintained at HM HIM" by plat im; the Italh containing
tin; tubes on a boiling water-kith.mpered steels.mulhod